1. Field of the Invention
The present invention relates to a thermal printer having a thermal head that functions as a printing device.
2. Description of the Related Art
A thermal dye sublimation printer will be described below as an example of a known thermal printer. First, a printing device will be described. An ink ribbon and a thermal head are arranged on a print surface of a recording sheet, and a platen roller is disposed on the back surface of the recording sheet. To print a desired image on the recording sheet, a plurality of heater elements arranged along a line on the thermal head are selectively caused to generate heat in accordance with image information while the thermal head is being pressed against the platen roller. Accordingly, ink on the ink ribbon is sublimated and transferred onto the recording sheet.
A device for conveying the recording sheet includes a paper feed roller, which is a driving roller that conveys the recording sheet, and a paper press roller that is arranged parallel to the paper feed roller and presses the paper feed roller with a certain tension. Accordingly, the recording sheet is conveyed while being nipped between the two rollers.
The conveying device is placed upstream or downstream of the printing device, and the recording sheet is continuously nipped between the rollers included in the conveying device during the process of printing on the recording sheet.
In color printing, an image of a single color is printed while the recording sheet is conveyed once. A full-color image is obtained by repeatedly driving the paper feed roller in forward and reverse directions and successively printing images of different colors on the recording sheet.
However, in the known printing method, the recording sheet must be continuously nipped between the rollers included in the conveying device during printing, as described above. Therefore, regions near a print start edge and a print end edge of the recording sheet cannot be printed on and margins are formed along these edges.
In order to solve this problem, an upstream conveying device and a downstream conveying device may be respectively arranged upstream and downstream of the printing device in the recording-sheet conveying direction. The recording sheet is conveyed using the upstream conveying device while a first half of a print region that is adjacent to the print start edge is being printed on, and is conveyed using the downstream conveying device while a second half of the print region that is adjacent to the print end edge is being printed on. Accordingly, marginless printing in which no margin is left on the recording sheet can be performed.
Japanese Patent Laid-Open No. 10-217516 discloses a structure for forming a color image over the entire region of a recording medium.
In the above-described print method for marginless printing, the printing device is disposed between the upstream conveying device and the downstream conveying device. Therefore, if the recording sheet cannot be smoothly passed from the upstream conveying device to the downstream conveying device, the conveying speed of the recording sheet varies. As a result, print defects like uneven density and color misalignment may occur in the printed image.
One of the reasons why the above-described problem occurs is deflection of the recording sheet between the upstream conveying device and the downstream conveying device. There are some factors between the upstream conveying device and the downstream conveying device that lead to the deflection of the recording sheet, and these factors are regarded as the causes of reduction of stability in the printing operation.